I think anger and fear are for losers and for the weak. These two emotions don't fit with SIth "survival of the strongest" behaviour and their winner mentality. Anger greatly diminishes ones focus and mental performance and consequently increases chance for errors. Winners have no reason to be angry while losers do. As for fear, fear is a powerful weapon so therefore the Sith should induce fear and terror into their enemies, not the other way around. A Sith who is afraid sounds pathetic and weak.
Fear is the way to the Dark-side.
Fear leads to Anger
Anger leads to Hatred
Hatred leads to Suffering